The present invention relates to a pressing method, in particular for obtaining hydraulic cylinders and high-pressure filters.
Nowadays, for the manufacture of these devices, methods of the known type such as, for example, hot-extrusion processes are used.
These hot-extrusion processes require the formation initially of a cylindrical metal part, such as, for example, a section of round bar or billet of suitable dimensions, cut to a length which is related to the volume of the part to be obtained.
The cylindrical part is heated to a temperature of about 1200xc2x0 C. in a furnace usually of the gas type.
The punching step is then performed: the cylindrical part, still hot, is placed in a die inside a press and upsetting is performed using a punch until the typical bell shape is formed.
The part is then extracted still hot, ready for carrying out an external drawing operation: this procedure envisages that the metallic material, which is drawn by means of a tool called a drawplate, is profiled according to the shape of the latter.
Upon leaving the drawplate, the part has a deformed cross section, which is smaller than that upon entry, and a smaller thickness of the side walls, resulting from the use of a suitable spindle or punch located coaxially with said drawplate.
From this drawing operation, a rough semifinished product having an external and internal surface finish with scales, grooves and other imperfections is obtained: the concentricity tolerances, for example, may vary from 10% to 30% of the thickness of said rough semifinished product.
After this hot-drawing operation, a cold-drawing operation is envisaged, preceded by the following three steps: an external turning operation is performed so as to give the bell part obtained from extrusion a concentric shape, followed by a sandblasting operation and by a process involving phosphating of the semifinished product.
Normally cold-drawing is performed with several passes: the number of these passes depends on the thickness of the part to be obtained; after each pass, an annealing, phosphating and sandblasting operation must be performed.
The main disadvantage of this hot-extrusion and cold-drawing process consists in the fact that it requires very large and costly machinery and long tool-changing times: the minimum amount of parts required in order for this hydraulic cylinder and high-pressure filter manufacturing method to be economically viable must thus be not less than one hundred tons of parts.
Another disadvantage of this method of the known type consists in the fact that the product obtained therefrom requires further machining operations in order to fall within the range of required tolerance limits.
Internal and external turning are thus performed, eliminating the layer of material having metallurgical characteristics altered by heating: in machining operations of this type about 15-20% of the material is lost.
Another method of the known type for manufacturing hydraulic cylinders and high-pressure filters consists in chip-forming machining of a metallic cylindrical part, such as a rolled solid steel bar.
Partial boring is then performed in an axial direction, followed by turning of the external and internal surfaces.
The main disadvantage of this method of the known type consists in the fact of having long programmed machining times which result in an increase in the cost of each individual part.
Another drawback which the method described here has is that the abovementioned chip-forming machining operations result in considerable wear of the cutting tools.
This wear becomes even greater if materials with a carbon content of less than 0.20% are used; these materials, moreover, since they have a low chip-forming capacity, create problems with regard to the use of hard-metal bits, with a consequent further increase in the machining times and costs.
A further disadvantage consists in the huge quantity of swarf and waste material since normally, during these chip-forming machining operations, more than 80% of the material used is lost.
The main aim of the subject of the present invention is therefore that of solving the technical problems mentioned, eliminating the drawbacks according to said prior art, and therefore devising a method which allows devices such as hydraulic cylinders and high-pressure filters to be obtained, using simple machinery with small dimensions and power requirements and low costs.
In connection with the abovementioned aim, another important object is to provide a method which involves short tooling-up times so as to be able to handle batches with a small number of parts.
Yet another important object is to provide a method which produces an optimum finish with low surface and concentricity tolerances so as not to require further machining operations.
Another object consists in obtaining a product which has optimum mechanical characteristics, often avoiding the need for further heat treatment.
Another object consists in obtaining the product without the need for intermediate annealing and sandblasting operations, which are very costly and wasteful.
Yet another object consists in providing a method which does not require, during manufacture of the cylinder, any welding.
Last but not least, an object is to provide a method which is simple and economical, said method producing a small amount of swarf.
The aim and the abovementioned objects, as well as others which will appear more clearly below, are achieved by a pressing method, in particular for obtaining cylinders and filters, to be carried out on a semifinished product advantageously consisting of a cylindrical part axially having a blind hole at one end and suitably treated by means of a first chemical surface treatment of the known type, which is characterized in that it comprises the following steps:
first pressing using a first punch acting over part of the depth of said blind hole, so as to define a first machined zone;
first sizing of said first machined zone using a second reverse-profile punch, so as to define a second machined zone;
second chemical surface treatment of said semifinished product thus obtained;
second pressing of said second machined zone using a third punch, so as to define a third machined zone with a desired thickness for said semifinished product thus obtained;
second sizing of said third machined zone using a fourth punch, so as to define a fourth flat contact zone, so as to obtain said cylinder.
Further characteristic features and advantages of the invention will emerge more clearly from the detailed description of a particular embodiment, illustrated by way of a non-limiting example in the plates of accompanying drawings, in which: